A spark plug boot which is also known as a Spark plug adapter is a connector which connects the output lead of HT coil and the spark plug. This HT connector serves as the safe heaven suppressing the radio frequency interference produced by the spark plug.

A faulty/cracked Spark plug connector leads to Idling instability, misfiring and hinders the ride-ability of the motorcycle affecting other parameters too.

A typical spark plug connector or an adapter consists of three parts. Spark plug housing, a ceramic resistor with a coil spring and a pointed screw setup.

1) The ceramic resistor functions as a suppressor which reduced the radio frequency interference. A stainless steel wire remains coiled inside connecting the the spark plug end and the HT coil end. The coil spring acts as a resistor as well as an inductor.

2) Pointed Screw setup for HT coil lead connection. The HT coil wire which are made up of heat resistant like silicon or (Ethylene Propylene Diene Monomer ) EPDM rubber as insulator with copper strings inside is tightly screwed onto this portion. As the wire penetrates the screw portion the copper strings expand and tight fits between the outer barrel and the leads making it more secure and firm.

As the vehicle is cranked the output of the HT coil is fed into the connector via the screw setup. Post suppressing the electronic interference, the voltage is further fed to the spark plug end

A faulty resistor or a broken/cracked outer insulating sleeve remains the key cause of failures. A broken sleeve leads to improper conduction and voltage drop which ultimately leads to in-sufficient spark across the spark plug terminals.

The failure cause of the SPK Plug adapter is heat. On heating the insulator cap gets cracked. This often occurs in non genuine parts.
Another reason being the resistor failure.
Boots like those of MSD's have hardly failed till date.
As a mech i have come across mechs who hit the boot on the ground to remove debris if any in the slots which i would term as one reason for failure. The sleeve is made of hard plastic which is subjected to develop cracks on such conditions. The cracks propagate on heating and vibrations, ultimately the boot fails.